Flow meters are known, for example, in which an electric resistivity probe depending on temperature is heated by a Joule effect while it is immersed in a stream of that fluid. Flow variations are detected as a function of variations in the electric resistance of the probe.
The flow meter of this type is the object, in particular, of patent GB No. 1 345 324. According to this patent, the heating energy for the probe is provided by a train of identical pulses separated by intervals which are sufficiently long to guarantee that the probe will be returned to its temperature of equilibrium with the fluid at the beginning of each pulse. Flow variations are detected by the variations recorded from one pulse to the other in the rate of change of the electric resistance of the probe during a pulse. This rate of change in resistance is given by the voltage variation derived from a thermistor immersed in that fluid. The slope of the voltage variation curve or the difference between the height of the front and rear sides of each pulse is detected. The voltage at the terminals of the thermistor is applied to a voltage-frequency converter the frequency of which decreases continuously as a function of the progressive decrease in voltage. The frequency which results for the duration of the pulse is then counted and the flow rate is deduced.
From the electronic point of view, the greatest part of the processing of the signal is analog in nature which leads to a relatively complex electronic circuit and which is therefore relatively expensive. In addition, the conversion of the voltage into a variable frequency covers a range of measurement which is rather small. This remark is, by the way, also true with regard to constant power heating pulses.
There are a large number of devices or processes designed to determine an instantaneous parameter of a fluid linked to the thermal exchange between that fluid and a probe immersed in that fluid. Mention may be made, for example, of patent applications FR No. 2 168 458, DAS No. 1 252 437, CH No. 491 375 as well as U.S. Pat. No. 4,068,526. All these documents relate to devices or processes for the determination of such a parameter on the basis of the measurement of an electric quantity characteristic of a first temperature followed, after a givena time interval, by the measurement of an electric quantity characteristic of a second temperature. Whatever the electric quantity thus measured may be, for example, voltage, this analog quantity must be converted to a digital quantity so that it can be used in a calculator programmed to determine this parameter. This digital analog conversion makes the processing circuit more complex and therefore more expensive. Now, there are a number of applications in which a system of electronic measurement could be considered, taking into account, in particular, advances in microcomputer techniques. In order for such a measurement to be carried out rationally and the processing circuit to be manufactured at a low price, in particular, for large consumption applications, it must of course be possible to introduce digital data directly into the calculator. Therefore, the solutions described in the above-mentioned documents are not adapted, without any modification, to processing by a calculator and require an entire interface which makes the circuit substantially more expensive.